EFFECTS OF HIGH PRESSURE ON FAR-INFRARED SPECTRA OF PARAELECTRIC KH2PO4 AND RBH2PO4

Using an opposed-anvil-type diamond cell, the infrared spectra of KH 2PO4, RbH2PO4, and monoclinic RbD2PO4 have been measured in the 100-1400-cm-1 region up to pressures of 60 kbar and at several temperatures. The room-temperature spectrum of RbH2PO4 demonstrates the existence of a new high-pressure phase where the PO4 tetrahedra are distorted and the protons are ordered. This phase does not occur in monoclinic RbD2PO4. The results with KH2PO4 are not as dramatic as RbH2PO4. The effects of high pressure in the 300-600-cm-1 region are similar to those obtained on cooling below the Curie temperature (Τe), and have been interpreted in terms of resonance interference between a multiphonon quasicontinuum and discrete states of the distorted PO4 groups.